Electrical conducting material



Feb. 17, 1942. R. o. GRISDALE ELECTRICAL CONDUCTING MATERIAL Filed Oct.10, 1935 2 Sheets-Sheet 1 FIG FIG. 3

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A T TORNEV Patented Feb. 17,1942

UNITE-D STATE s: PATENT OFFICE I ELECTRICAL CONDUCTING MATERIAL Richard0. Grisdale, New York, N. Y.,' assignor to Bell Telephone Laboratories,

Incorporated,

This invention relates to electrical conducting materials and, moreparticularly, to such conducting materials having anon-linearvoltagecurrent characteristic.

An object'of this invention is to improve the methods of manufacture andthe characteristics of non-ohmic conducting materials. Another object isto produce non-ohmic conducting materials having reproduciblecharacteristics and undergoing no irreversible changes in normal usebecause of change in temperature or continued application of excessvoltages.

Stillanother object is to produce conducting materials showing non-ohmiccharacteristics at potentials between a few hundredths of a volt and afew hundred volts.

1 A further object is to improve methods of depositing films ofsemi-conducting or insulating materials on metallic or semi-conductingplates, members or granules. r X

Other and furtherobjects will be apparent from the detailed descriptionwhich follows hereinafter.

In accordance with this invention, a non-ohmic conductor is provided byassociating a pair of plates, discs or members of a metal or of asemi-conducting material separated by a thin film of an insulatingmaterial. If the separated members are of the same material, thenonohmic characteristic is symmetrical; if'the members are of difl'erentmaterials, the characteristic is asymmetrical. A non-ohmic conductor isobtainable, also, by coating individual granules of a conductingmaterial with an-insulating film and, thereafter, compressing or bondingthem together, metallic contact being made to the surfaces of the unitthus formed. When a bond is various electrical conducting materials ordevices in accordance with the invention.

Materials or devices whose conductivities increase as the potentialapplied to them increases, that is, materials in'which the currentincreases more rapidly than linearly with voltage, are defined asnon-ohmic conductors. If the conductivity is electronic, the conductoris called an electronic non-ohmic conductor. In general, theconductivities of electronic non-ohmic conductors are independent of thewave shape, of the frequency, and, except for efforts due to Jouleheating, of theduration of application of the applied potential, beingdependent only .on the meanmagnitude "of this potential. There are,however, electronic non-ohmic conductors whose conductivities arefunctions of the polarity of the appliedpotential, such materials beingreferred to as asymmetric non-ohmic conductors. With the latterconductors, capable of rectifying alternating current, it is generallyfound that, while for a given polarity of applied potential, theconductivity'is non-ohmic as above defined, the conductivity for apotential of opposite polarity may decrease over certain ranges as thepotential is increased, that is, the current may increase less rapidlythan linearly with applied potential.

It has been found that materials ordinarily regarded as insulatorsbecome conductors when used in thin films between two conducting bodiesin such a way that the voltage gradient in them is great, and that,except in cases where measurements havebeen made near the point ofdielecemployed, it is preferable that it should not react chemicallywith or tend to dissolve the compound of which the film is composed.

tion will be obtained from the detailed descrip- A more completeunderstanding of this invention which follows taken in conjunction withthe appended drawings, wherein:

Fig. 1 shows a non-ohmic conducting device embodying the invention;

Fig. 2 shows another embodiment of the invention in the form of acompressed granular aggregate;

Fig. 3 shows still another embodiment of the invention in the form of acompressed, bonded granular aggregate;

I Fig. 4 shows another embodiment of the invention; and --Figs. 5 to 14show characteristic curves" of tric breakdown, non-ohmic conduction isobserved only at solid-solid interfaces, and that the so-called body orintrinsic conductivity of solids is strictly ohmic.

It has been usually assumed that non-ohmic conduction occurs only atinterfaces between semi-conductors orbetween semi-conductors and metals,a semi-conductor being defined as a material'whose conductivity is muchless than that of a metal but much greater than that of an insulator. Ithas been found, however, that such an assumption is not correct, andthat apparently the sole requirement for non-ohmic conduction is thatthe surfaces of two conducting bodies be separated by a physically thinfilm of a material possessing a low intrinsic specific conductivity.When, furthermore, bodies of the same intrinsic conductivity areseparated by such a film, the arrangement exhibits symmetrical non-ohmicconductivity; when thefilm separates bodies of widely differentconductivities, for example, a

metal and a semi-conductor, the conductivity will be asymmetric, thearrangement acting as a rectifier for applied alternating potentials.

While it has been observed that physically thin films of materials, suchas adsorbed gases, waxes,

'oils and other organic substances forming interfacial layers betweenconducting bodies give rise to the effects mentioned hereinabove, suchmaterials are in many cases too unstable to be employed in a non-ohmicconductor to be used in an electrical circuit. This invention,therefore, is directed to the production and utilization of thin filmsof inorganic materials, such as metallic oxides and silicates that arelmown to be stable. Both symmetric and asymmetric nonohmic conductorshave been produced from a variety of materials.

There are at least two distinct methods of producing symmetricalnon-ohmic conductors. The first of these is the creation of a thin filmof a material having a low specific conductivity as an interfacial layerbetween parallel surfaces of two bodies having the same specificconductivity. The second method is to coat granules of a conductingmaterial with such thin films and then to compress an, aggregate of suchfilm-coated granules between a pair of metallic electrodes, or to bondsuch granules in a matrix which holds them in contact, metallic contactbeing made to the surfaces of the bonded aggregate. It can be shownstatistically that when granular aggregates are employed, it is notnecessary that the specific conductivity of each granule'be the same asthat of every other one, since, if the size of the granules be smallcompared to the dimensions of the aggregate, the probability that thereare an equal number of contacts exhibiting asymmetric conductivityineither senseisunity. Thus, while symmetric non-ohmic conductivity canbe obtained by combining large numbers of asymmetric non-ohmicconductors in a random manner, asymmetric non-ohmic conductivity canarise only when there is but one interface or when all interfaces arepresent in an ordered system.

Fig. 1 shows a symmetric non-ohmic conductor or device, designatedgenerally 20, comprising a pair of layers or plates 2|, 2| of asemiconductor, such as boron carbide or silicon carbide separated by athin'film 22 of an insulating or other material of low specificconductivity, such as lead oxide, the outer surfaces of the layers 2|being engaged by metallic contacts 23, 23, applied thereto, forinstance, by spraying. The contacts 23 may be omitted if a suitablemetal is substituted for the semi-conductor layers.

Such a metal may be gold, platinum, nickel,

chromium, copper or other metal or alloy and in place of the lead oxide,thallous oxide, silicon dioxide, lead borate or lead silicate may beemployed as the film of insulating material. A symmetric non-ohmicconductor comprising outer layers of gold and an intermediate film oflead oxide has a voltage-amperage characteristic such as is shown inFig. 5.

Fig. 2 shows a symmetric non-ohmic conductor in which granular materialis employed. Granules 24, spherical or irregular in shape, arecompressed between the externally threaded terminal or contact plates 25that engage with the internally threaded bore.or passage 26 in a sleeveor housing 21 of insulating material. Each granule is of asemi-conductor, for instance, boron carbide or silicon carbide, with acontinuous coating or film of lead oxide. The film, alternatively, maybe of lead silicate, thallous oxide,

lead borate or silicon dioxide. If desired, the granule may be of quartzor sillimanite coated with a metal such as gold, the gold being coatedwitha film of an as lead oxide. In Fig. 3, the granules are showncompressed and bonded together in a body or member 28, as explained ingreater detail hereinafter, metallic contacts 29 engaging the outeropposed surfaces of the body.

* An asymmetric non-ohmic conductor in accordance with this invention isshown in crosssection by Fig. 4. It comprises a layer or plate 30 of ametal, such as platinum, having on one surface a thin film 3| of aninsulating material, for example, lead oxide, a. layer or plate 32 of asemi-conducting material, such as cuprous oxide, being in intimatecontact with the film. The voltage-amperage characteristic of such aconductpr is shownby Fig. 6. Other metals that may be employed aresilver, gold, nickel, chromium and iron; other semi-conductors, siliconcarbide, boson carbide, iron oxide, cupric oxide and'a mixture ofsilicon carbide, clay and carbon.

Instead of having only one layer or plate in the device or conductor ofFig. 4 of a semi-conducting material, each of the layers separated 'bythe film may be constituted of semi-conducting materials'of diflerentspecific conductivities. For example, the conductor may comprise a filmof lead oxide deposited on a plate of boron carbide, contact being madeto the film surface by means of a composition of silicon carbide, clayand carbon. The voltage-amperage characteristic for such a conductor isshown by Fig. 7L A similar result can be obtained by forming thin filmsof lead oxide between surfaces of two other semi-conductors such assilicon carbide and iron oxide, iron oxide and cuprous oxide, or ironferrite and a composition of silicon carbide, clay and carbon. Otherinsulating films, such as silicon dioxide, thallous oxide, lead borate,lead silicate, or aluminum oxide, can be used in place of the leadoxide.

It has been found that so long as the compound of which the film iscomposed possesses a low specific conductivity is stable, and isstoichiometrically pure, its composition is relatively comes possible..fsentially that of unimportant. An example of this is shown in Fig. 8,where curves A, B and C are the characteristics of an aggregate ofgranules of silicon coated with films of lead oxide, thallous oxide, andsilicon dioxide, respectively. The chemical purity of such films isimportant in that it has been found that the presence of small amountsof impurity in a normal insulating compound renders it a semi-conductorpossessing an appreciable conductivity having a positive temperaturecoeflicient. It would appear that it is essential that the film have avery low inherent conductivity, it having been observed that thenonohmic properties 'of such an interface decrease greatly or vanishentirely when the film has a conductivity of the order of that normallyassociated with semi-conductors.

The production, therefore, of non-ohmic conductors from a wide varietyof materials be- The problem thus becomes esproducing the thincontiguous insulating films of high chemical stability at interfacesbetween conducting bodies. There are numerous methods available fortheir production. If the bodies are metallic, atmospheric or anodicoxidation will yield, in many cases, thin films of the metallic oxides.Heating granular metallic insulating material such silicon in air willproduce a silicon oxide film on the granules. .The characteristic of abonded aggregate of such granules is shown by Rig. 9. Thin layers ofmetals can be produced on surfaces by vaporization, chemical reduction,coagulation of colloidal suspensions, electrolysis, or sputtering; andthese metal surfaces can be transformed to insulating .films byoxidation. Spherical quartz granules were coated with gold by sputteringand subsequently coated with lead oxide by the method describedimmediately hereinafter. Compressed between metallic electrodes orcontact plates, an aggregate of these granules gave the characteristicshown by 10.

In some cases, metallic sulfides can be deposited in thin films on cleansurfaces by coagulation of colloidal suspensions resulting from thedecomposition of complex sulfur bearing salts; and these sulfides can beconverted to oxides by heating in air. A film of lead sulfide can beformed on gold by immersing a gold plate or granules in a colloidalsuspension of lead sulfide,

formed from a solution of lead acetate and thiourea by the addition ofsodium hydroxide solution. The sulfide film is subsequently converted tothe oxide by heating in air. The thermal decomposition of volatileorganic or inorganic metallic compounds in many cases yields thin filmsof the metals or their oxides. For example, films of nickel can be madeby heating the surface to be coated in nickel carbonyl, or iron films byheating the surface in iron carbonyl, or films of silicon by heating thesurface in silicon hydride.

It is clear that non-ohmic conductors can be produced by creating oninsulating bases successive layers of metals or semi-conductors andinsulating materials. While films of materials such as oxides, silicatesand borates have been stressed, there appears to be no reason to believethat films of other chemical compounds could not be used provided theyare chemically stable and of low specific conductivity.

The above-described method of making both symmetric and asymmetricnon-ohmic conductors also furnishes a means of imparting magneticproperties to such conductors, this being accomplished by using magneticsubstances, such as iron, cobalt, nickel, or other alloys, or magneticoxides or ferrites, as the bases on which the films are created. Itfurnishes a means for controlling the film thickness and hence thenon-ohmic properties, as well as a means of studying separately theeffects due to the film and those due to the base on which the film orbarrier layer is deposited, this being hitherto impossible. It permitsof the control of each stage, therefore, in the production of non-ohmicmaterials.

In the form of granular aggregates, non-ohmic conductors arenot wellsuited to use in electrical circuits. Consequently, it is necessary tobond these aggregates in some mechanically stable matrix in such a wayas to provide intimate and permanent contact between granules. Thenonohmic conductor would be in the form, therefore, of the device ofFig. 3. Among the bonds that have been employed are the thermal settingand thermal plastic organic resins, cements, glasses,

silicates, borates and various ceramic compositions. The process ofbonding comprises intimately mixing the bond with the non-ohmicconducting granules, pressing the mixture into a unit of the desiredsize and shape, and heating to a temperature to cause the bond to softenand to coalesce. To retain its original characteristics, he bondingmatrix must undergo no ical or phase change, or be subject to plasticflow. In addition to these requirements, the bond must not reactchemically with or tend to dissolve the compound of which the insulatingfilm covering the granules is composed. Since the solubility of the filmin. a bond will depend upon temperature, the film must be insoluble inthe bond at the temperature used in causing the bond to coalesce.

It has been determined that when an aggregate of film coated granulesexhibiting non-ohmic conduction is bonded in a ceramic matrix in whichthe compound of which the film is'composed is insoluble, that is, inwhich the film component exists as a separate phase, the non-ohmicproperties of the granular aggregate are retained in the completeddevice. When the matrix is such that it reacts or forms a solid solutionwith the film component, the non-ohmic properties. of the granularaggregate are either greatly diminished o; disappear entirely. It isfound in extreme cases that when such is the case the specificconductivity of the bonded aggregate approaches that of the granularmaterial on which the insulating films were originally produced. Theseresults have been obtained from studies on ceramic bonds composed of theoxides of calcium, silicon, magnesium, and aluminum, which were employedas matrices for granules of silicon carbide whose surfaces were coatedwith thin films of oxide'of silicon. It was observed that those bonds inwhich silicon dioxide existed as a separate phase yielded non-ohmicmaterials, while those in which'silicon dioxide was soluble yieldedeither materials less non-ohmic or practically ohmic possessing muchgreater conductivities.

An example of the effect of the bond on the non-ohmic conductingproperties of a non-ohmic conductor is the following. Granules ofsilicon carbide were heated in air to produce a film of silicon dioxideon their surfaces. These granules, when subsequently compressed betweenmetallic electrodes, had the non-ohmic characteristics shown in curve Aof Fig. 11. When these granules were bonded with a mixture of the oxidesof calchemcium, silicon, and aluminum in which silicon dioxide wasinsoluble, the non-ohmic characteristics shown in curve B were obtained.When similarly treated silicon carbide granules were bonded with amixture of calcium, aluminum, and silicon oxides, in which silicondioxide was soluble, the characteristics were those shown in curve C. Inthis case, the conductor has lost most of its non-ohmic characteristicsand befound in the case of granules of silicon coated silica and leadoxide are insoluble have been successfully employed as matrices forcoated grancreased while its non-ohmic properties are decreased: Byvarying the conductivity of the matrix by the addition, for example, ofpowdered conducting substances, wide variations in the conductivities ofthe bonded ag tes can be realized, such variations being invariablyascciated with changes in the magnitude of the nonohmic properties.Thus, ceramic materials containing powdered conducting may be used asbonds for non-ohmic granular ggregates provided neither the ceramicmatrix nor the conducting substance added to it reacts with or tends todissolve the film component. In like manner, semi-conductors maythemselves be employed as bonding matrices subject to these conditions.As a consequence, the nature of the bonding matrix is to a large extentdefinitive of the resultant non-ohmic properties of non-ohmic granularaggregates embedded in this matrix.

In non-ohmic conductors composed, for instance, of granules of siliconcarbide embedded in ceramic, inorganic or organic matrices, as thespecific conductivity of the material at any given voltage is increasedby suitable treatment during its production, the non-ohmic properties ofthe conductor are reduced in magnitude. That is, for any given non-ohmicmaterial there exists an inverse relationship between the value of theconductivity and the magnitude of the non-ohmic properties. Sinceconductors of widely varied conductivities possessing non-ohmicproperties of essentially the same magnitude are desirable, it would beadvantageous to be able to vary the conductivities of the conductorsover wide limits without changing the degree of departure from Ohms law.

It has already been pointed out that rectification or asymmetricnon-ohmic conduction is obtained when two conducting bodies of differentspecific conductivities are separated by aphysicallythinfilmofamatelialpossessinga low specific conductivity. Thisis true even in the case of two semi-conductors separated by such afilm. It has been found, furthermore, that when two identical substancesare separated by such a film, the difierential conductivity of thesystem is a function of the specific conductivity of the substancesbetween which the film exists. By difierential conductivityis meant thevalue of the conductivity as derived from the.

tivity ofthefilminthe first case would be greathowever, essentiallyslope of the current-voltage characteristic; it will be referred to asthe absolute specified voltage. g.

In the case of an asymmetric non-ohmic conductor, the conductivities foropposite polarities of the same applied potential diifer because of thisefieet. The conductivities in opposite directions are flmctions of theelectron concentrations and distributions in the two substances and ofthe ease with which electrons from either substance pm through the filmto the other. Consequently, if identical films serve as the interfacesbetween two bodies of the, same metal in one case and between two bodiesof the sam conductivity at any e semi-conductor in another, theabsoluteconduclute conductivities are dependent on .those of iron,copper, or

er than that in the second. The essential point, however, is that, if ineach case the film has the same thickness, the non-ohmic properties ofthe two conductors are virtually identical even though their absoluteconductivities are widely separated. The properties of non-ohmicmaterials, therefore, can be put under two general classifications: (1)The non-ohmic properties of symmetrical non-ohmic conductors aredependent on the specific conductivity of the film component and on thefilm thickness; (2) the absothe specific conductivities of the bases onwhich the films are deposited. It is apparent that by creating identicalfilms on the surfaces specific conductivities, the absoluteconductivities of the resulting non-ohmic conductors will be directfunctions of the specific conductivities of these materials.

It has been found that either metals or semiconductors can be employedequally well as bases on which thin films'are deposited in makingnon-ohmic conductors. For example, granules of metallic silicon may becoated with thin films of silicon dioxide by being heated in air, and,thereafter, bonded in a phenol condensation product by it with 20percent by weight of moulding powder and mo ding. The voltagecurrentcharacteristic for t bonded aggregate is shown by curve A of Fig. 13. Asa further example", granules of silicon carbide may be coated with fihnsof silicon dioxide by heating them in air, and similarly bonded. Thevoltagecurrent characteristic for the latter bonded'aggregate is shownby curve B of Fig. 13. The absolute conductivities of the resultantconductors differ by a factor of about twenty thousand which is the sameas the ratio between the specific conductivities of silicon and siliconcarbide. The non-ohmic properties of the two are,

the same.

Films of lead oxide may be deposited on granules of silicon carbide,boron carbide and silicon. The absolute conductivities of the resultantnon-ohmic conductors are in direct relation to the specificconductivities of the bases on which the lead oxide was deposited, thenonohmic properties being the same for all these, as shown by thecharacteristic curves of Fig. 14. When bonded in a matrix of leadsilicate and lead oxide, the granular aggregates evidenced the samebehavior.

Other conducting bases may be used with equally good results. Amongthose readily available are the carbides of metals such as tungsten,titanium, and zirconium, metallic oxides such as nickel, and othercompolmds, such as ferrites. Semi-conductors of widely varied specificconductivities can be produced from mixtures of oxides, ferrites andcarbides; or, in general, by adding controlled amounts of impuritieseither in the form of separate compounds or as elements to normallynon-conducting materials. The bases can be made, also, by creating filmsof'semi-conductors on the surfaces of metallic, ceramic or othergranules. By forming thin films of materials low specific conductivitieson the surfaces of these bases, non-ohmic conductors or widely variedconductivities equivalent non-ohmic properties can be produced. Inaddition, by properly choosing the bases, magnetic or other propertiescan be imparted to the non-ohmic materials.

of materials of varied While this invention has been disclosed withreference to various specific embodiments thereof, it will be understoodthat the scope of the invention is to be considered as limited by theap-2. An electrical conductor having non-ohmic characteristics for apotential range from a few volts to a few hundred volts, said conductorcom prising conductive granules, each of which is completely coated witha thin film of a material having a low conductivity such as is normallyassociated with insulators, and a bond for bonding the granules togetherand which does not react chemically with the film.

3. An electrical conductor having non-ohmic characteristics for apotential range from a few volts to a few hundred volts, said conductorcomprising conductive granules, each of which is completely coated witha thin film of a material having a low conductivity such as is normallyassociated with insulators, and a ceramic bond for bonding the granulestogether and which does not react chemically with the film.

4. An electrical conductor comprising an ag gregate of granules ofconductive material each completely coated with a thin film of silicondioxide.

5. A non-ohmic conductor comprising granules of conductive material, athin continuous insulating film on each granule, and a thermosettingresin bond intimately uniting, said particles.

6. A non-ohmic conductor having a range of conductivity for use inelectrical circuits which comprises granules of a material having thedesired range of conductivity, a thin continuous insulating film on eachof said granules, and a bond for intimately bonding the granules into aunit.

7. A non-ohmic electrical conductor comprising an aggregate of granulesof silicon each coated with a thin film of silicon dioxide andintimately bonded in a ceramic matrix.

8. A non-ohmic conductor comprising granules the surface of each ofwhich is conductive, a thin continuous insulating film on each granuleand a ceramic bond for intimately unitin the filmed granules.

9. A non-ohmic conductor having high con-, ductivity comprising granulesof a metal of high conductivity, a thin continuous insulating film oneach of said granules, and a ceramic bond intimately uniting said filmedgranules.

10. A non-ohmic conductor having a low con- 4 ductivity comprisinggranules of semiconducting material, a thin continuous insulating filmon each of said granules, and a ceramic bond intimately uniting saidfilmed granules.

composed of disintegrated portions of the crystalline structure andconsisting primarily of silica free from aportion of the carbon normallypresent in silicon ,carbide surfaces.

13. A device for controlling the flow of current comprising a body ofgrains having crystalline silicon carbide bodies and having surfacesresulting from the decomposition of portions of the crystallinestructure into silica and carbon and the elimination of a portion of theliberated carbon;

14. A device for controlling the flow of electric current including acurrent-carrying mass of crystalline, valve action silicon carbidegranules having integral oxidized surfaces containing oxide, principallysilica, on the granules.

15. A device according to claim 14 comprising a body of grains havingcrystalline silicon carbide cores and having surfaces composed ofdisintegrated portions'ofv the crystalline structure converted intooxide consisting principally of silica.

RICHARD O. GRISDALE.

